Fluteless taps



United States Patent Otitiee 3,248,747 Patented May 3, 1966 Claimspriority, application Great Britain, Nov. 1, 1962,

37,134/ 62 2 Claims. (Cl. 10-152) Fluteless 4taps are taps which do notcut threads but act by deforming the wall of a cylindrical bore withoutdeliberately removing any material. For this purpose a fluteless tap hasa thread with a number of circumferentially spaced radially relievedparts between which there are accordingly circumferentially spacedcrests. At their maximum radial distance from the axis of the tap thesecrests have profiles, as seen in section planes lying along the axis ofthe tap, corresponding to t-he shape of the thread which is to be formedby the tap. For convenience we refer to the diameter of the tap, meaningthe diameter of the thread which will be formed by the tap, that is tosay -twice the maximum radius of the crests from the axis of the tap.

According to the present invention a tap has at the leading end aprimary thread Zone which has circumferentially spaced crests at aconstant radial distance which is less than the radial distance of thecrests on the remainder of the tap but is greater than half of lthecrests on the remainder of the tap; that is -to say, the radius of theprimary thread Zone is smaller than the radiusA of the remainder by anamount less than half the radial depth of the crests on the remainder.The thread on the tap has a smooth transition from the primary threadZone to the remainder of the thread, which remainder may be termed thesecondary thread Zone.

In use when the tap is screwed into a bore of the appropriate diameter,the primary thread zone on the tap partially forms the required threadin the bore, and the thread in the bore is completed by the secondarythread Zone. As the primary thread zone can enter relatively easilyint-o the bore (as compared with a tap with a thread of constant radiusall the Way to the end) and is of a constant radius, it ensures that theaxis of the tap is accurately aligned with the axis of the bore. Toensure accurate alignment the primary thread Zone preferably has atleast three turns of thread.

An example of a tap according to the present invention is shown in theaccompanying drawings. In the drawings:

FIGURE l is a fragmentary side view showing the threaded part of thetap; and

FIGURE 2 is a cross section, which for convenience is taken in a helixthrough 'thencentre of the helical valley between two adjacent turns ofthe thread.

The tap has a primary thread zone 2 at its leading end, and a secondarythread zone 4. There are about three turns of the primary thread Zone,and the transition from the primary zone to secondary zone takes placethrough one turn of the thread.

The secondary thread zone has an addendum A, that is to say, half theradial depth `of the final thread which is formed by the secondary zonethread. The diameter of the crests of the primary thread zone is equalto the diameter of the secondary thread zone minusthe addendum of thesecondary thread zone.

The secondary thread zone has a pitch diameter P2. It will be seen thatthis is smaller than the crest diameter of the primary thread zone. Theprimary thread zone has a pitch diameter P1.

As shown in FIGURE 2, the tap has four circumferentially spaced crests8. Each crest is at the junction of a steeply relieved leading part 10and a progressively relieved trailing part 12. The tap diameter is thedimension D, It will be seen that the outer edge of the lead-ing part 10is substantially a straight line passing through the crest tangentiallyto a circle M which has the minimum diameter of the tap. The outer edgeof the progressively relieved trailing part 12 extends from the crest toa point at about the junction of the circle M with the outer edge of theleading part of the adjacent crest. The relief of diameter, that is tosay the difference between the dimension D yand the diameter of thecircle M, is shown slightly exaggerated in FIGURE 2 for the sake ofclarity.

The thread form shown in FIGURE 2 is the subject of patent applicationNo. 30,995/ 60.

I claim:

1. A lluteless tap having, in axial sequence, a leading end, a primarythread zone, a Itransition thread Zone, and a secondary thread zone;each of the primary and secondary thread zones consisting of a pluralityof helical turns, each turn, viewed axially, having a plurality ofcircumferential-lyvspaced crests, all the crests of the primary threadZone lying a-t a uniform rst radius from the axis of the tap, and allthe crests of the secondary thread Zone lying at a uniform second radiusfrom the axis of the tap; all the thread zones having turns which,considered in planes containing the axis, are of the same profile; Ithefirst radius being less than the second radius by an amount not eX-ceeding half the radial depth of the crests on second thread zone; thetransition thread zone consisting of about one spiral turn which linksthe adjacent turns of the primary and secondary thread zones; and theprofile of the turns having an apex at its radially outermost extremity,whereby the part of the primary thread zone adjacent t-o the leading endwill exert -a rst thread forging action upon insertion into a bore andthereby form a primary thread, the remainder of the primary thread Zonewill maintain the tap in axial alignment in the bore, and the transitionthread zone will exert a second thread forging action upon the primarythread and thereby enlarge the primary thread into secondary thread, andthe remainder of the secondarythread Zone will smooth the secondarythread.

2. A iiuteless tap according to claim 1, in which the rst radius issubstantially equal to the second radius minus one quarter of the radialheight of Ithe prole.

References Cited by the Examiner UNITED STATES PATENTS 84,411 ll/l868Clark et al. 10-141 2,325,627 8/1943 Neilson 10-l4l 2,807,813 10/1957Welles 10-152 2,991,491 7/1961 Welles 10-15?- ANDREW R. JUHASZ, PrimaryExaminer.

1. A FLUTELESS TAP HAVING, IN AXIAL SEQUENCE, A LEADING END, A PRIMARYTHREAD ZONE, A TRANSITION THREAD ZONE, AND A SECONDARY THREAD ZONE; EACHOF THE PRIMARY AND SECONDARY THREAD ZONES CONSISTING OF A PLURALITY OFHELICAL TURNS, EACH TURN, VIEWED AXIALLY, HAVING A PLURALITY OFCIRCUMFERENTIALLY SPACED CRESTS, ALL THE CRESTS OF THE PRIMARY THREADZONE LYING AT A UNIFORM FIRST RADIUS FROM THE AXIS OF THE TAP, AND ALLTHE CRESTS OF THE SECONDARY THREAD ZONE LYING AT A UNIFORM SECOND RADIUSFROM THE AXIS OF THE TAP; ALL THE THREAD ZONES HAVING TURNS WHICH,CONSIDERED IN PLANES CONTAINING THE AXIS, ARE OF THE SAME PROFILE; THEFIRST RADIUS BEING LESS THAN THE SECOND RADIUS BY AN AMOUNT NOTEXCEEDING HALF THE RADIAL DEPTH OF THE CRESTS ON SECOND THREAD ZONE; THETRANSITION THREAD ZONE CONSISTING OF ABOUT ONE SPIRAL TURN WHICH LINKSTHE ADJACENT TURNS OF THE PRIMARY AND SECONDARY THREAD ZONES; AND THEPROFILE OF THE TURNS HAVING AN APEX AT ITS RADIALLY OUTERMOST EXTREMITY,WHEREBY THE PART OF THE PRIMARY THREAD ZONE ADJACENT TO THE LEADING ENDWILL EXERT A FIRST THREAD FORGING ACTION UPON INSERTION INTO A BORE ANDTHEREBY FORM A PRIMARY THREAD, THE REMAINDER OF THE PRIMARY THREAD ZONEWILL MAINTAIN THE TAP IN AXIAL ALIGNMENT IN THE BORE, AND THE TRANSITIONTHREAD ZONE WILL EXERT A SECOND THREAD FORGING ACTION UPON THE PRIMARYTHREAD AND THEREBY ENLARGE THE PRIMARY THREAD INTO SECONDARY THREAD, ANDTHE REMAINDER OF THE SECONDARY THREAD ZONE WILL SMOOTH THE SECONDARYTHREAD.